Process and apparatus for drying micro-organisms, in particular edible yeasts

ABSTRACT

An aqueous suspension obtained by culturing a micro-organism, especially a fodder or edible yeast, is treated to give a dried product by concentrating it as by filtration, bringing it to a solids content at which it can be pelleted, forming it into pellets, and drying the pellets by a fluidized bed technique at a temperature of 50* to 400* C. The solids content of the concentrate can be further increased as required by a pre-drying step at a temperature of 50* to 400* C., or preferably by recycling and mixing with it part of the dried material.

Field of Search ..34/8, l0, 12, 57 A, 57 C 0 United States Patent [is]3,660,908 Bardot 51 ay 9, 11972 [54] PROCESS AND APPARATUS FOR ,[56]References Cited DRYING MICRO-ORGANISMS, IN

- UNITED STATES PATENTS PARTICULAR EDIBLE YEASTS 2,677,608 5/1954 McKayet al ..34/l0 [72] Inventor: Paul Bardot, Paris, France PrimaryExaminer-John J. Camby [73] Assignee. Societe Pour L Equipement DesIndustries I Chimiques speichim, Paris France Attorney -Stevens, Davis,M ller 84 Mosher [22] Filed: Sept. 15, 1970 [57] ABSTRACT [21] Appl.No.: 72,334 An aqueous suspension obtained by culturing amicro-organism, especially a fodder or edible yeast, is treated to givea dried product by concentrating it as by filtration. bringing it toForeign PP Priority Data a solids content at which it can be pelleted.forming it into pellets, and drying the pellets by a fluidized bedtechnique at a Sept France "6931305 temperature of 50 to 400 C. Thesolids content of the concentratecan be further increased as required bya pre-drying [52] U.S.Cl. ..34/8, 34/10 Step at a temperature of to C orpreferably by Cl "F26b 5/08 recycling and mixing with it part of thedried material.

15 Claims, 1 Drawing Figure PROCESS AND APPARATUS FOR DRYING MICRO-ORGANISMS, IN PARTICULAR EDIBLE YEASTS The present invention relates tothe drying of cultured micro-organisms, in particular edible yeasts,which are available in the form of an aqueous suspension of creamyconsistency having a solids content which is most commonly of the orderof 13 to 15 percent by weight, though it can reach about 18 percent.

It is known that the cells of yeast are rich in proteins and also inso-called essential" amino-acids, that is to say in amino-acids whichthe primary organism is not capable of synthesizing. The edible yeastsare generally considered to be the best sources of proteins which allowthe formulations of foodstuffs intended for human and animal consumptionto be corrected.

For a yeast to be an excellent component of a foodstuff ration it isnecessary for its proteins to be digestible. However, the still livingcells are protected by a cellulosic envelope or membrane which theintestinal juices cannot attack and which must be hydrolysed to renderthe yeast digestible.

The digestibility is measured by the pepsin method, which allows theproportion of digestible protein contained in the yeast to be defined;in order to hydrolyze the cellulosic membrane of the cell at leastpartially, a heat treatment either preceding or following concentrationis generally used.

It has already been proposed to dry the yeast either on rolls or byspraying after concentration.

Drying on rolls is applicable to small-scale production, generally lessthan tons of solids per day, or about 500 kg of yeast powder per hour.The cream is passed into a steam-fed roll drier, which dries it in theform of a film which is detached from the roll by means of a doctorblade. in this process, the yeast is thermolyzed when it comes intoactual contact with the roll, at the same time as it is dried.

In the processes which involve spray drying and are suitable for thelargest-scale production, concentration by evaporation has beenproposed. In this case, the yeast cream of for exam ple about percentsolids content issuing from the centrifuge separator passes throughvarious heaters operated by the action of the concentrator, and isheated in these. It thereafter passes into a pasteurizer or thermolizerand it is then directed onto the bundles of tubes of the concentrationapparatus, where it is concentrated by evaporation in vacuo.

The product which leaves the concentrator generally contains from 22 to23 percent of solids. At this degree of concentration, the thermolyzedcream is relatively liquid. This cream is thereafter dehydrated in aspray drying apparatus.

instead of using evaporation before spraying, it has also been proposedto achieve concentration by a filtration step. In this case, the creamcontaining for example 15 percent of solids is charged onto a continuousrotating filter, which produces a cake or a paste containing about 25 to27 percent of solids. This paste passes through a heating apparatuswhich raises it to 8085 C over the course of about 5 minutes, whilerendering it fluid and hence capable of being pumped.

The yeast which has thus been rendered liquid is spraydried.Spray-drying generally demands very costly installations of very largedimensions.

The present invention is designed to give a very digestible yeast, inparticular a yeast capable of being digested to about 80 to 90 percentby pepsin, in large amounts, at reduced cost and with better yield thanhas been general heretofore.

The invention is applicable generally to the treatment of anymicro-organism cultured for industrial purposes, in particular forfoodstuff purposes, but more especially to edible yeasts (includingfodder yeasts), regardless of how the cells are cultured, e.g. ondistillers wash, cane sugar molasses or sugar beet molasses, milk serumor petroleum products. For this reason, and for convenience ofpresentation, specific reference will be made to yeast, but it must beunderstood that the invention is not limited thereto.

In one aspect the invention consists in a process for drying a cultureof a micro-organism obtained as an aqueous suspension, which comprisesconcentrating the suspension and bringing it to a solids content atwhich it can be formed into granules or pellets, forming it intogranules or pellets, and drying the granules or pellets by afluidization technique.

Preferably the initial concentration is effected by filtration using arotary filter, but other devices, e.g. a filter press or a centrifuge,can be usedif desired.

To bring the material to a solids content at which it can be formed intogranules or pellets (hereinafter termed the "pelleting point") it isusually preferable to add to the immediate product of the concentrationstep (hereinafter referred to as filter cake") a suitable proportion ofa dried product similar thereto but of higher solids content. Mostadvantageously this added material is recycled dried product.

The pelleting point will vary with the particular micro-organism beingtreated. Thus among the yeasts the pelleting point for the Saccharomycesis 28 30 percent, for the Candidas is 35 38 percent, and for the Torulasis 56 57 percent. For convenience and for safety it is usuallypreferable to bring the solids content to at least a little above thepelleting point for the micro-organism in question. If the pelletingpoint is not reached, the product is more or less tacky or gelatinous,and cannot usefully be converted into coherent granules or pellets onconventional pelleting machinery.

It is also possible to raise the solids content of the filter cake bygiving it a pre-drying treatment, whether or not a propor tion of driedmaterial is also added. However, drying at this stage presents certaintechnical difficulties, largely because water has to be removed fromwithin the cells (intra-cellular water) as well as from between cells(interstitial water), and living and killed cells differ in the easewith which they give up water, and in any case are not in the samephysical condition for handling.

For a predrying operation without addition of dried material, it isdesirable to divide the filter cake into fragments or crumbs, forexample by passing it under a rotating spindle provided with teeth, andto treat the fragments at a temperature and for a time which aresufficient for the pelleting point to be at least reached and preferablyexceeded, and also for the cells to be killed without damage to theessential amino acids, in particular lysine, whose content in soybeanflour is relativel low.

Suitable temperatures for pre-drying are within the range 50 400 C. Foreach temperature within this range one can readily determine byexperiment the minimum drying period required to reach the pelletingpoint for the micro-organism in question. Thus, heating the filter caketo C. for a few minutes as previously proposed removes hardly any waterand would be quite insufficient, and as a rule pre-drying at such atemperature requires about a to 1 hour. If the pre-drying is to besupplemented by the addition of dried material, shorter drying times orlower temperatures within the said range can naturally be used.

The pre-drying can be effected by a static or fluidized bed technique,while in another method the fragments or crumbs of filter cake aretransferred to a perforated conveyor belt which moves, during therequired pre-drying time, through a tunnel swept by a gas stream at theselected temperature.

The gas in which the granules or pellets are suspended during thefluidisation drying can be air, depleted in oxygen if circumstancesdemand it, or an inert gas such as nitrogen, and this gas'can beintroduced into the fluidization zone at a temperature of 50 to 400 C. Atemperature of the order of C., say 135 to C., is satisfactory in allrespects.

The invention comprises also an apparatus suitable for car rying out theprocess described above, which comprises in combination a yeast filter,a device for forming the filter cake into granules or pellets, a devicefor drying said granules or pellets by fluidization, and means forfeeding the said granules or pellets thereto. Optionally it comprisesalso means for recycling part of the material which has left the dryingmeans to the filter cake before it is formed into granules or pellets.

The invention will now be described, by way of example only, withreference to the accompanying drawing, which is a schematicrepresentation of an apparatus in accordance with the invention.

Referring now to the drawing, the apparatus comprises a continuousrotating filter l which at 2 receives a yeast cream of about 15 percentsolids content. The web of concentrated yeast paste which leaves thefilter is passed by means of an inclined surface 3 to a continuousgranulator 5, of which the rotor and the drive motor are schematicallyshown at 50 and 5b respectively. As will be seen later, the paste ispreferably intimately mixed with some drier yeast powder delivered at 6and in this case, as the drawing shows, it is then diverted into a feedhopper 7 which ends at the inlet of a continuous mixer 8, comprising amixing rotor 8a and drive motor 8b.

The granulator 5 is so mounted that the material issuing from it entersa device for drying fluidization, designated overall by 9, andcomprising a suspension chamber 9a above a perforated vibrating table9b, a chamber 9c for the introduction of hot gas under this table, andan outlet 9d, for dry yeast powder. The table 9b is operativelyconnected to vibrator 10.

In this example, the hot gas is air drawn from the atmosphere through afilter 11 and forced by means ofa fan 13 across a heater l4 and into thechamber 90, from where it rises across the perforated table 9b.

The hot air which has flowed turbulently in the chamber 9a and has theredried the yeast particles while killing its cells, carries the finest ofthese particles through a pipe line 11 to a cyclone 32 surmounted by asuction fan 33. From the bottom of the cyclone, a pipe line 34 passesthe dry particles which have been separated off to the outlet 9d, whererotating sluices 15 for removing the main mass of the dry product arealso provided.

In a preferred embodiment, a pipe line 16 starts from the outlet 9d andends in a separator for solids, such as a cyclone 17 combined with asuction fan 18, the solid material from the cyclone being passed at 6into the hopper 7. A pipe line 19 leads the gas from the fan 18 back tothe cyclone 32.

As shown, the apparatus can be completed by an elevator conveyor 20, forexample of the screw type (screw 20a and driving motor 20b), asemi-automatic balance 21 for weighing and sack-filling (sack 22), aconveyor belt 23 for the sacks, and a machine 24 for sewing up thesacks.

In this apparatus all the constituent parts can be of conventionaldesign, and do not require further description. The rotating filter 1can, if desired, be replaced by a filter press or a centrifuge.

The recycled dry yeast powder which is delivered at 6 will normally beintroduced in such proportions relative to the paste coming from thefilter that the mixture has a solids content of over 27 percent, theprecise value varying with the micro-organism involved as alreadydescribed. The temperature and the flow of air through the chamber 9acan be so adjusted that the granules coming from the apparatus 5 arerapidly raised to a temperature of about 80 C., at which the live cellsare killed.

On issuing from the drying apparatus the granules will typically have amoisture content of about 4 percent, and can be packaged as such orground to give a finer powder.

The installation described offers various advantages.

As in the known process using filtration followed by spraydrying, theamount of water removed by filtration exceeds that removed byevaporation, but the fluidization drying requires considerably less heatthan spray-drying; for example, if steam heating is used, a low pressureboiler (less than 6 kg/cm) can be substituted for a medium pressureboiler l0 kglcm The continuous enrichment of the paste issuing from thefilter with recycled dry material is easily controlled so as to bringthe material as a whole to a moisture content, and hence to aplasticity, which is suitable for good granulation or pelletmg.

The heat yield of the drying process with air at 150 C., for example,generally exceeds 60 percent, though it is necessary to reach atemperature of 400 C. for a similar yield in spraydrying. This isimportant, since the spontaneous ignition point of yeast powder is about400 C.

Loss due to entrainment of dried material in the moist air issuing fromthe drying apparatus is considerably reduced, because the finesoriginating from compacted granules do not exceed 5 percent by weight ofthe dry product and have a size between and 200 microns, making itpossible to use cyclones of large diameter, which are less expensivethan the cyclones for spray-dried yeast powders; in this latter case,cyclones of small diameter are required because the average size of thefine powder particles passing through them is between 10 and 60 microns.

For withdrawing the same amount of water from the yeast cream so as tobring it to the pulverulent stage, an installation for drying byfluidization costs very significantly less than an installation forspray-drying, usually 30 to 50 percent less, depending on the particularcase.

The running cost also is less because:

a. At equal heat yield, drying by fluidization will require calories ata low temperature (e.g. about 0), instead of at 400 C., as is inspray-drying.

b. The loss of powder carried away in the moist drying air ispractically negligible, and

c. There is less water to remove in drying in the filtration processthan in the evaporation process (70 percent of water to be removed bythe first process, as against 78 to 80 percent in the second).

Typical micro-organisms to which the invention can be applied includeTorula utilir and T.famala, Rhodotorula sp., Torulopsis sp. andT.utilis, Candida lipolytica, CJropicalis, C. rugosa, C.sp., C.guillermondii, C. roburta, C. pelliculosa, C. in- !ermedia and C.arborea, Trichonospore culaneum, Trichosporum capilanicum, Endomycesvernalis, Bacillus paraffinicus, Methanomonas, Pseudomonas species andMicrococcur cerrfcans, as well as Aspergillii and Penicillia.

I claim:

1. Process for drying a culture of a micro-organism obtained as anaqueous suspension, which comprises concentrating the suspension andbringing it to a solids content at which it can be formed into granulesor pellets, forming it into granules or pellets, and drying the granulesor pellets by fluidization technique.

2. Process according to claim 1, in which the initial aqueous suspensionis concentrated by filtration or centrifuging.

3. Process according to claim 2, in which the micro-organism is a fodderor edible yeast.

4. Process according to claim 3, in which the micro-organism is asaccharomyces yeast and the product obtained by the concentration isbrought to a solids content of 28-30 percent or higher before it isformed into granules or pellets.

5. Process according to claim 3, in which the micro-organism is aCandida yeast, and the product obtained by the concentration is broughtto a solids content of 35-38 percent or higher before it is formed intogranules or pellets.

6. Process according to claim 3, in which the micro-organism is a Torulayeast, and the product obtained by the concentration is brought to asolids content of 56-57 percent or higher before it is formed intogranules or pellets.

7. Process according to claim 1, in which the fluidisation and dryingare effected with a stream of gas at a temperature of 50 400 C.

8. Process according to claim 7, in which the fluidization and dryingare effected with a stream of gas at a temperature of 135 C.

9. Process according to claim 1, in which there is added to the productof the concentration step sufficient of a dried product similar theretobut of higher solids content to raise its solids content at least to thelevel at which formation of granules or pellets becomes possible.

10. Process according to claim 3, in which sufiicient of the driedproduct is recycled and mixed with the product of the concentration stepto raise its solids content at least to the level at which formation ofgranules or pellets becomes possible.

combination a filter, a device for forming solid material from thefilter into granules or pellets, means for drying said granules orpellets by a fiuidization technique, and means for feeding the granulesor pellets to the said drying means.

15. Apparatus according to claim 14, comprising also means for recyclinga part of the material which has left the drying means and adding it toand mixing it with the solid material from the filter before it isgranulated or pelleted.

t l '1 Q t

2. Process according to claim 1, in which the initial aqueous suspensionis concentrated by filtration or centrifuging.
 3. Process according toclaim 2, in which the micro-organism is a fodder or edible yeast. 4.Process according to claim 3, in which the micro-organism is asaccharomyces yeast and the product obtained by the concentration isbrought to a solids content of 28-30 percent or higher before it isformed into granules or pellets.
 5. Process according to claim 3, inwhich the micro-organism is a Candida yeast, and the product obtained bythe concentration is brought to a solids content of 35-38 percent orhigher before it is formed into granules or pellets.
 6. Processaccording to claim 3, in which the micro-organism is a Torula yeast, andthe product obtained by the concentration is brought to a solids contentof 56-57 percent or higher before it is formed into granules or pellets.7. Process according to claim 1, in which the fluidisation and dryingare effected with a stream of gas at a temperature of 50* - 400* C. 8.Process according to claim 7, in which the fluidization and drying areeffected with a stream of gas at a temperature of 135* - 165* C. 9.Process according to claim 1, in which there is added to the product ofthe concentration step sufficient of a dried product similar thereto butof higher solids content to raise its solids content at least to thelevel at which formation of granules or pellets becomes possible. 10.Process according to claim 3, in which sufficient of the dried productis recycled and mixed with the product of the concentration step toraise its solids content at least to the level at which formation ofgranules or pellets becomes possible.
 11. Process according to claim 1,in which the solids content of the product of the concentration step isincreased by giving it a pre-drying treatment.
 12. Process according toclaim 11, in which the product of the concentration step is fragmented,and the fragments are pre-dried at a temperature of 50* - 400* C. 13.Process according to claim 12, in which the fragments are pre-dried by afluidization technique.
 14. Apparatus suitable for drying a culture of amicro-organism obtained as an aqueous suspension, comprising incombination a filter, a device for forming solid material from thefilter into granules or pellets, means for drying said granules orpellets by a fluidization technique, and means for feeding the granulesor pellets to the said drying means.
 15. Apparatus according to claim14, comprising also means for recycling a part of the material which hasleft the drying means and adding it to and mixing it with the solidmaterial from the filter before it is granulated or pelleted.